In theory, Train 750 on the Fairmount Line is a commuter’s dream, cutting across a congested city — from Hyde Park to South Station — in just 30 minutes.
Problem is, it has a maddening habit of not showing up.
For five straight days earlier this month, then again last Tuesday, the 6:25 a.m. commuter rail train was canceled, a head-shaking pattern of failure that left riders, many of them from lower-income neighborhoods, at a loss and doomed to be late to work.
Worse, their train was often running fine. It was just being used somewhere else.
“What they do is they take our train and then give it to a longer-distance line, which is not fair,” said Deborah Phillips, rolling her eyes in frustration Wednesday morning as she waited at a train stop in Mattapan to get to work. “We buy our passes; we pay our taxes. It’s not right.”
So far this month, Keolis Commuter Services, the company that runs the commuter rail, has canceled Train 750 six times, following four cancellations in September, according to Keolis. On the entire Fairmount Line, which runs through Roxbury and Dorchester on its way downtown, Keolis says 17 trains have been canceled this month. That appears to be more than almost any other line, a review of Keolis’s Twitter alerts shows.
In August, the problem was even worse for the Fairmount Line. Nineteen trains were canceled, several during rush hour.
The Fairmount Line is a modest route, with eight stops over about 9 miles. But for residents, many of whom rely on public transit, it’s a vital connection meant to offset the lack of subway access.
The MBTA has spent hundreds of millions of dollars to improve the line, as part of a promise to expand transit options in some of Boston’s less prosperous communities.
But its cancellations, which dwarf the number of canceled trains on several other lines that serve wealthier, suburban neighborhoods, speak to broader problems in the public transit system, riders and transportation specialists say.
It raises questions not only about equal treatment in a system that primarily caters to suburban riders, but also whether Keolis can cover its network of routes on any given day. The Haverhill line appeared to have a similar number of trains canceled, and only a few lines had no cancellations at all, according to the commuter rail’s Twitter alerts.
Rafael Mares, a vice president at the Conservation Law Foundation, an environmental advocacy group that supports expanded public transportation, said the cancellations signify a larger failure.
“The Fairmount Line riders are bearing the brunt of the bigger problems — that there aren’t enough coaches,” he said. “The line is being used essentially as a backup line.”
In a statement, Keolis’s new general manager, David Scorey, apologized to riders and blamed the frequent cancellations on a “short-term” shortage of available coaches, “which we are working to rectify as quickly as possible.”
Leslie Aun, a Keolis spokeswoman, also said in an e-mail that four cancellations stemmed from a pedestrian being struck on the tracks of the Franklin Line, which uses some of the same tracks. Fairmount Line trains typically have just 50 to 75 riders — compared with nearly 900 riders on busy lines — and there is bus service available at every Fairmount station, she added.
“As a result, when there is an equipment shortage for whatever reason, this train is often the one that gets canceled in order to protect trains with much higher ridership,” she said.
Stephanie Pollack, the state’s transportation secretary, said in a prepared statement that the line’s recent performance is “unacceptable,” and that the state is meeting with Keolis about having enough trains available to cover all the lines each day.
In October, the dropped trips represented about 2 percent of all scheduled Fairmount Line trains, but Pamela Bush, the lead community organizer for the Greater Four Corners Action Coalition, a Dorchester group that advocates for transportation and other services, said she has watched in frustration as the cancellations mount, particularly at rush hour when people need trains the most.
“If we’re depending on this transportation, we shouldn’t have to find another way home and take the long way home,” she said. “We came a long way, and we don’t want to go backward. There should be equitable transportation for all.”
Aun said that ridership on the Fairmount Line has held steady since 2013, but Mares and Bush said more commuters would take the line if they could count on it.
Bush said commuters have also noticed more cancellations when the New England Patriots play preseason weeknight games, when the MBTA runs an extra train to Gillette Stadium. Commuters grew so weary of the missing trains that Mares said he took their complaints to an MBTA commuter rail official this fall.
“It’s a huge problem if you can’t rely on it,” he said. “You decide you’re going to use the longer trip on a regular basis, because at least you know it’s going to be provided.”
Commuters do not get refunds or discounts for canceled trains. Sometimes, the MBTA has provided a replacement bus to Fairmount riders, but often it offers no alternatives.
When Train 750 rolled into the Morton Street station last Wednesday, Bernadette Macon-Bell took stock of the 13 other people waiting to board: the 20-something in a suit, a handful of high schoolers, and other commuters she sees every day, many going to jobs downtown.
There were fewer kids on their way to school than usual, another rider said. Their parents must have realized they couldn’t count on it.
As Macon-Bell scrolled through her text messages to find all the cancellation notifications from Keolis, she did her best to look on the bright side.
“The train’s on time when it comes,” she said. When it doesn’t, she takes a bus, the Red Line, and the Silver Line bus to work at Logan Airport, a trek that sometimes takes an hour and 15 minutes.
At first, Sharol Knox, one of Train 750’s most loyal riders, was willing to give Keolis the benefit of the doubt, figuring the train had mechanical problems. But when she e-mailed her complaints to customer service, employees explained that they sometimes needed to borrow the train for busier lines.
That made her angry, and when she read the news that the MBTA had forgiven nearly $839,000 in fines against Keolis from the winter of 2015, she got angrier.
“They cancel ours and they say there are mechanical issues, and it’s not even our train that has a mechanical issue,” said Knox, who started taking the line in 2005.
“They write me and say, ‘It’s because we have other modes, and we don’t have enough people,’ but that’s not right. Fix the train.”
The Fairmount Line represents a promise from the MBTA to some of the city’s underserved neighborhoods. As the state planned for the Big Dig, the MBTA agreed to renovate the line so that commuters would have a better option than crowded, slower buses.
After the MBTA added two stations in 2013 and lowered prices to subway rates to build up ridership, the line had a “good little run,” Knox said. But like most lines, it struggled during the record-breaking winter of 2015, and cancellations grew more common as this summer wound to a close.
Joe Pesaturo, an MBTA spokesman, pointed out that Keolis is fined nearly $5,400 for every canceled train. Since taking over in 2014, the company has been fined about $14 million for everything from late trains to broken toilets.
For Knox, that offers little consolation. The problem seems to defy explanation, she said, and Keolis doesn’t seem to be held responsible.
“What do they care? They’re still getting paid,” Knox said. “They’re getting fines reduced, so why should they care?”
John R. Ellement of the Globe staff contributed to this report. Nicole Dungca can be reached at email@example.com. Follow her on Twitter @ndungca.